Radical reorientation dynamics studied by positive-muon avoided level crossing resonance

Spin polarised positive muons substituted for a hydrogen nucleus in organic radicals are used as spin labels to sense reorientational motion of the radicals in orienting environments. The effect of uniaxial motion on the spectrum of the muonated cyclohexadienyl radical in a benzene single crystal and of its hexamethyl derivative in polycrystalline hexamethylbenzene is demonstrated. Isotopic spectra for cyclohexadienyl radicals adsorbed on SiO₂ are explained by translational diffusion around the 7 nm grain. The potential of the technique to study transient radical intermediates of processes in heterogeneous catalysis is discussed.